The present invention relates to a method of measuring a profile of an object under the water in a non-contact and non-destructive manner and with high precision by ultrasonic waves, and a measurement apparatus for carrying out the method.
A profile of an object which is made of a hydrophilic, soft material and is wetted in use, such as a soft contact lens (hereinafter merely referred to as "lens" in the description), needs to be measured with the object kept in a condition smaller to actual use. Here, the "profile of an object" to be measured includes not only the contour of a section of the object but also the thickness, curvature radius, etc., of the object throughout the description unless otherwise defined. Conventionally, to measure the profile, a lens, taken out of water, is cut and the longitudinal section thereof is magnified using a projector, or a lens is observed while immersed under water by an optical instrument.
In the fomer method wherein the lens is taken out of water for measurement, the lens must be cut and accordingly is unsalable, and since water evaporates from the lens, the lens is deformed and thus accurate measurement cannot be ensured. Furthermore, cutting and measurement of the lens are carried out manually, and therefore measurement error varies greatly depending on the skill in measurement.
On the other hand, the latter method in which a lens under water is measured by an optical instrument is disclosed in Japanese Patent Laid-Open Publication No. 52-70849. In this method, a lens is placed on a lens table having a predetermined effective diameter, with a convex surface thereof directed upward, and a focusing plate is moved so that graduation "A" marked thereon is on a level with the vertex of the convex surface of the lens, while watching the vertex of the lens through a window formed in a side wall of a water tank by an optical system. Subsequently, the lens is moved up from the lens table by a probe so that the vertex thereof is on a level with graduation "B" of the focusing plate, whereby the base curve of the lens is obtained from the amount of movement of the measurement part. In this method, the measurement is carried out with the lens immersed under the water and therefore no deformation occurs in the lens. However, since only limited information (i.e., the effective diameter of the lens table, the distance between the graduations "A" and "B" of the focusing plate, and the distance over which the probe is moved) is available, the overall profile of the lens cannot be measured.
There have been propose a method in which ultrasonic waves are irradiated to an object to be measured, and the difference in time between reflected wave signals reflected at the surface of the object and reflected wave signals reflected at the surface of a base on which the object is placed is used to obtain the thickness of the object, a method in which ultrasonic waves are made to enter an object at the critical angle, and the attenuation of reflected waves produced as the result of leakage of elastic surface waves propagated along the surface of the object by the incident energy issued to obtain the thickness of the object (e.g., Japanese Patent Laid-Open Publication No. 60-120204), etc. In these methods, however, the ultrasonic transducer is fixed at a predetermined position relative to the object to be measured, and accordingly only thickness information at a certain point of the object can be obtained and it is not possible to obtain a thickness distribution of the object.